Thin film transistor for supplying power to element to be driven

ABSTRACT

An EL element ( 50 ) having an organic emissive layer or the like between the anode and cathode is used as an element to be driven, and an element driving TFT ( 20 ) for controlling the current supplied to the EL element ( 50 ) and a compensation thin film transistor ( 30 ) having an opposite conductive characteristic as the element driving TFT ( 20 ) are provided between the EL element ( 50 ) and the power supply line VL. With this structure, variation in the current supplied to each EL element ( 50 ) is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electroluminescent display device,and in particular, to transistors constructing the circuit structure inthe pixel section of an electroluminescent display device.

2. Description of the Related Art

An electroluminescence (hereinafter referred to as EL) display devicewhich uses an EL element which is a self-illuminating element as anillumination element in each pixel has attracted a strong interest as analternative display device for a display device such as a liquid crystaldisplay device (LCD) and a CRT because the EL display device hasadvantages such as thin width and low power consumption, in addition tothe advantage of being self-illuminating. Such an EL display device hasthus been researched.

In particular, there is a high expectation for an active matrix type ELdisplay device in which a switching element such as, for example, a thinfilm transistor for individually controlling an EL element is providedfor each pixel and EL elements are controlled for each pixel, as a highresolution display device.

FIG. 1 shows a circuit structure for one pixel in an active matrix typeEL display device having m rows and n columns. In the EL display device,a plurality of gate lines GL extend on a substrate in the row directionand a plurality of data lines DL and power supply lines VL extend on thesubstrate in the column direction. Each pixel has an organic EL element50, a switching TFT (first TFT) 10, an EL element driving TFT (secondTFT) 20, and a storage capacitor Cs.

The first TFT 10 is connected to the gate line GL and data line DL, andis turned on by receiving a gate signal (selection signal) on its gateelectrode. A data signal which is being supplied on the data line DL atthis point is then held in the storage capacitor Cs connected betweenthe first TFT 10 and the second TFT 20. A voltage corresponding to thedata signal is supplied to the gate electrode of the second TFT 20 viathe first TFT 10. The second TFT 20 then supplies a current,corresponding to the voltage value, from the power supply line VL to theorganic EL element 50. In this manner, the organic EL element in eachpixel is illuminated at a brightness based on the data signal, and adesired image is displayed.

The organic EL element is a current-driven element which is illuminatedby supplying a current to an organic emissive layer provided between acathode and an anode. The data signal output onto the data line DL, onthe other hand, is a voltage signal with an amplitude corresponding tothe display data. Thus, conventionally, in order to accuratelyilluminate the organic EL element by such a data signal, in an organicEL display device, a first TFT 10 and a second TFT 20 are provided ineach pixel.

The display quality and reliability of the organic EL display devicesdescribed above remain insufficient, and the characteristic variationsin the first and second TFTs 10 and 20 must be dissolved. In particular,reduction in characteristic variation in the second TFT 20 forcontrolling the amount of current supplied from the power supply line VLto the organic EL element 50 is desired, because such variation directlycauses variation in the illumination brightness.

Moreover, it is preferable to construct the first and second TFTs 10 and20 from a polycrystalline silicon TFT which has quick operation speedand which can be driven by a low voltage. In order to obtain apolycrystalline silicon, an amorphous silicon is polycrystallized bylaser annealing. Because of various reasons such as, for example, energyvariation in the irradiating laser at the irradiation surface, the grainsize of the polycrystalline silicon is not uniform. When grain size isnot uniform, in particular around the TFT channel, there is a problem inthat the on-current characteristic or the like of the TFT may also vary.

SUMMARY OF THE INVENTION

The present invention is conceived to solve the above problem, and oneobject of the present invention is to provide an active matrix typeorganic EL panel capable of illuminating each illumination pixel at auniform brightness by alleviating the characteristic variations of theTFT which controls the organic EL element.

In order to achieve at least the object recited above, according to thepresent invention, there is provided a device comprising anelectroluminescence element having an emissive layer between a firstelectrode and a second electrode; a switching thin film transistor whichoperates by receiving a gate signal at its gate, for reading a datasignal; and an element driving thin film transistor provided between adriving power supply and the electroluminescence element, forcontrolling the supply of power from the driving power supply to theelectroluminescence element based on a data signal supplied from theswitching thin film transistor, wherein a compensation thin filmtransistor having an opposite conductive characteristic as the elementdriving thin film transistor is provided between the driving powersupply and the element driving thin film transistor.

With such a compensation thin film transistor with an oppositeconductive characteristic, variation in the characteristic shift in theelement driving thin film transistor can be absorbed, and, thus, overallvariation among individual transistors can be alleviated. Variation inillumination brightness of the electroluminescence element due to thecharacteristic variation can thus be prevented.

According to another aspect of the present invention, it is preferablethat the compensation thin film transistor is a diode connectedtransistor disposed between the driving power supply and the elementdriving thin film transistor.

With such a structure, it is possible to compensate for thecharacteristic variation in the element driving thin film transistorwithout supplying a designated control signal to the compensation thinfilm transistor.

According to another aspect of the present invention, it is preferablethat, in the display device, the element driving thin film transistorcomprises a plurality of thin film transistors connected to each otherin parallel.

According to yet another aspect of the present invention, it ispreferable that the element driving thin film transistor comprises aplurality of thin film transistors connected between the driving powersupply and the electroluminescence element and in parallel to eachother; and the compensation thin film transistor is respectivelyprovided between the plurality of thin film transistors connected inparallel and the driving power supply.

In this manner, by providing a plurality of element driving thin filmtransistors in parallel, the overall influence on the characteristic ofthe transistors connected in parallel can be alleviated even whencharacteristic variation occurs in an individual transistor. Because ofthis, it is possible to supply current with small variation to the ELelement. In addition, by also providing a plurality of compensation thinfilm transistors, the influence of the characteristic variation in theindividual transistors on the characteristic of the overall pixeltransistor can be reduced, which facilitates illumination of the ELelement at a uniform brightness.

According to another aspect of the present invention, the semiconductordevice can be used for an active matrix type display device wherein eachof the pixels arranged in a matrix form comprises a switching thin filmtransistor, an element driving thin film transistor, a compensation thinfilm transistor, and the element to be driven which is the displayelement.

According to another aspect of the present invention, in thesemiconductor device, it is preferable that the element driving thinfilm transistor and the compensation thin film transistor are placed sothat the channel length direction of the thin film transistors are alongthe extension direction of the data line for supplying the data signalto the switching thin film transistor.

According to another aspect of the present invention, in thesemiconductor device or display device, it is preferable that theelement driving thin film transistor is formed so that its channellength direction is along the scan direction of a line pulse laser forannealing the channel region of the transistor.

In this manner, by coinciding the channel length direction of theelement driving thin film transistor and the scan direction of the laserannealing, the difference in the transistor characteristics from theelement driving thin film transistors for supplying power to otherelements to be driven can be reliably reduced.

In laser annealing, the laser output energy tends to vary. The variationincludes a variation in the pulse laser within an irradiation region andvariation among the shots. In many cases, the element driving thin filmtransistor which is used for a semiconductor device such as, forexample, an active matrix type display device, is designed so that thechannel length is significantly greater than the channel width. Byplacing the element driving thin film transistor along the longer sideof the pixel region or forming the element driving thin film transistoralong the column direction or the extension direction of the data line,the channel length of the element driving thin film transistor caneasily be increased to a sufficient length. By setting the scandirection of the laser to almost coincide with the channel lengthdirection of the element driving thin film transistor, that is, bysetting the scan direction of the laser so that the longitudinaldirection of the laser irradiation region crosses the channel in thewidth direction, the device can easily be adjusted so that the entirechannel region of one element driving thin film transistor is notsimultaneously annealed by a single shot. This can be easily achievedby, for example, setting the channel length of the element driving thinfilm transistor to be longer than one moving pitch of a pulse laser.Thus, in a case where a plurality of elements to be driven is formed onthe same substrate and a plurality of element driving thin filmtransistors for supplying power to the plurality of elements is formed,it is possible to laser anneal the active layers of the thin filmtransistors by a plurality of shots, resulting in the transistors to besubjected more uniformly to the energy variation among the shots andreliable averaging of the characteristics among the thin filmtransistors. In this manner, for example, in an organic EL displaydevice which uses an organic EL element as the element to be driven,which uses an organic compound as the emissive layer, variation in theillumination brightness among the organic EL elements provided fordifferent pixels can be significantly reduced.

According to another aspect of the present invention, in thesemiconductor device, it is preferable that the channel length directionof the element driving thin film transistor does not coincide with thechannel length direction of the switching thin film transistor.

The switching thin film transistor is placed near the section where theselection line for selecting the transistor and the data line forsupplying a data signal cross each other. In many cases, the switchingthin film transistor is placed so that its channel length direction isapproximately parallel to the extension direction of the selection line.In such a case, by placing the element driving thin film transistor sothat its channel length direction is different from that of theswitching thin film transistor, the channel length of the elementdriving thin film transistor can easily be increased.

According to another aspect of the present invention, it is preferablethat the element to be driven is an organic electroluminescence elementwhich employs an organic compound as an emissive layer. Although such anorganic EL element has high brightness and wider selection ranges forthe illumination color and material, because the organic EL element iscurrent driven, variation in the amount of supplied current causes avariation in the illumination brightness. By using the circuit structureof the pixel or placement as described above, it is possible to easilymaintain uniformity of the supplied current. In addition, by employingthe placement and structure of the contact points as described above,the aperture ratio can be increased and the element layer such as theemissive layer can be formed on a flat surface, and a more reliableelement can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a circuit structure for one pixel in anactive matrix type organic EL display device.

FIG. 2 is a diagram showing an example circuit structure of one pixel inan active matrix type organic EL display device according to a firstembodiment of the invention.

FIG. 3 is a diagram showing the I-V characteristic of a TFT.

FIGS. 4A and 4B are diagrams showing the effects obtained by the circuitstructure of the present invention and by a circuit of conventionalstructure.

FIG. 5 is a diagram showing another circuit structure of one pixel in anactive matrix type organic EL display device according to the firstembodiment of the invention.

FIG. 6 is a diagram showing yet another circuit structure of one pixelin an active matrix type organic EL display device according to thefirst embodiment of the invention.

FIG. 7 is a diagram showing still another circuit structure of one pixelin an active matrix type organic EL display device according to thefirst embodiment of the invention.

FIG. 8 is a diagram showing planer structure of the active matrix typeorganic EL panel according to the first embodiment of the presentinvention with the circuit structure shown in FIG. 7.

FIGS. 9A, 9B, and 9C are diagrams respectively showing the crosssectional structure along the lines A—A, B—B, and C—C of FIG. 8.

FIGS. 10A and 10B are respectively a planer diagram and a crosssectional diagram of one pixel of the active matrix type organic ELpanel according to a second embodiment of the present invention.

FIG. 11 shows another example of a planer structure of one pixel of theactive matrix type organic EL panel according to the second embodiment.

FIG. 12 is a planer diagram of one pixel of the active matrix typeorganic EL panel according to a third embodiment of the presentinvention.

FIG. 13 shows another example of a planer structure of one pixel of theactive matrix type organic EL element according to the third embodiment.

FIG. 14 shows yet another example of a planer structure of one pixel ofthe active matrix type organic EL panel according to the thirdembodiment.

FIGS. 15A and 15B are diagrams respectively showing the cross sectionalstructure and planer structure of the contact section between the activelayer 16 of the second TFT and the anode 52 of the organic EL element50.

FIGS. 16A and 16B are diagrams respectively showing the cross sectionalstructure and planer structure of the contact section between the activelayer 16 of the second TFT and the anode 52 of the organic EL element 50according to a fourth embodiment of the present invention.

FIG. 17 is a diagram showing another example of a cross sectionalstructure of the contact section between the active layer 16 of thesecond TFT and the anode 52 of the organic EL element 50 according tothe fourth embodiment.

FIG. 18 is a diagram showing a further example of a cross sectionalstructure of the contact section between the active layer 16 of thesecond TFT and the anode 52 of the organic EL element 50 according tothe fourth embodiment.

FIG. 19 is a diagram showing yet another example of a cross sectionalstructure of the contact section between the active layer 16 of thesecond TFT and the anode 52 of the organic EL element 50 according tothe fourth embodiment.

FIG. 20 is diagram showing another example of a cross sectionalstructure of the contact section between the active layer 16 of thesecond TFT and the anode 52 of the organic EL element 50 according tothe third embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention (hereinafter referred tosimply as embodiments) will now be described referring to the drawings.

First Embodiment

FIG. 2 shows a circuit structure of one pixel in an active matrix typeEL display device having m rows and n columns according to a firstembodiment of the present invention. As shown in FIG. 2, each pixelcomprises an organic EL element 50, a switching TFT (first TFT) 10, anelement driving TFT (second TFT) 20, and an storage capacitor Cs, and isconstructed in a region surrounded by a gate line GL extending in therow direction and a data line DL extending in the column direction. Inthe first embodiment, a compensation TFT 30 having the conductivecharacteristic opposite of that of the second TFT 20 is provided betweenthe power supply line VL and the second TFT 20. The gate and either thesource or the drain of the compensation TFT 30 are connected to providea diode connection. The diode is connected in the forward directionbetween the power supply line VL and the second TFT 20. Thus, thecompensation TFT can be operated without supplying any designatedcontrol signal.

The first TFT 10 is turned on by receiving a gate signal at its gate.When the first TFT 10 is turned on, the data signal supplied to the dataline DL is held at the storage capacitor Cs connected between the firstand second TFTs 10 and 20, and the potential at one electrode of thestorage capacitor Cs becomes equal to the data signal. The second TFT 20is provided between the power supply line VL and the organic EL element(the anode of the element) 50, and operates to supply a currentcorresponding to the voltage value of the data signal applied to itsgate, from the power supply line VL to the organic EL element 50. In theexample shown in FIG. 2, an nch-TFT which is capable of high-speedresponse is used for the first TFT 10 and a pch-TFT is used for thesecond TFT 20.

An nch-TFT having a polarity opposite that of the second TFT 20 is usedfor the compensation TFT 30. When the I-V (current-voltage)characteristic of the second TFT 20 is varied, the I-V characteristic ofthe compensation TFT 30 is varied in the opposite direction, thuscompensating for the characteristic variation of the second TFT 20.

FIG. 3 shows I-V characteristics of an nch-TFT and a pch-TFT which usepolycrystalline silicon for the active layer. In the nch-TFT, when thevoltage applied to the gate exceeds a predetermined positive voltage(+Vth), the current value is rapidly increased. In the pch-TFT, on theother hand, when the voltage applied to the gate becomes less than apredetermined negative voltage (−Vth), the current value is rapidlyincreased. In an nch-TFT and a pch-TFT formed on the same substrate, forexample, when the threshold value, +Vth, of the nch-TFT is varied by anincrease, that is, shifted to the right in FIG. 3, the threshold value,−Vth, of the pch-TFT is shifted by about the same degree to the right ofFIG. 3. In contrast, when the threshold value, +Vth, of the nch-TFT isshifted to the left, the threshold value, −Vth, of the pch-TFT is alsoshifted to the left. For example, due to a variation in themanufacturing condition or the like, when −Vth of the pch-TFT used forthe second TFT 20 of FIG. 2 is shifted to the right, in a conventionaldevice, the amount of current supplied to the organic EL element 50under the same condition would immediately be reduced. However,according to the first embodiment, the amount of current flowing fromthe compensation TFT 30, which is provided between the second TFT 20 andthe power supply line VL and which is constructed from an nch-TFT, isincreased.

According to the first embodiment, as shown in FIG. 2, a second TFT 20and a compensation TFT 30 of opposite polarity are provided between thepower supply line VL and the organic EL element 50. The two TFTs arethus balanced. That is, the amounts of current flowing from these TFTsalways compensate for each other. In the circuit structure of the firstembodiment, due to the presence of the compensation TFT 30, the maximumcurrent value which can be supplied to the organic RL element 50 is lessthan that in the conventional circuit structure shown in FIG. 1 whichdoes not have the compensation TFT 30. However, because theidentification sensitivity of human eyes at a high brightness issignificantly lower than that at an intermediate brightness, a smallreduction in the maximum current value which can be supplied does notsignificantly influence the display quality. On the other hand, becausethe second TFT 20 and the compensation TFT 30 adjust the current flowingfrom each other in each pixel, the variation in the amount of currentsupplied to the organic EL element 50 among the pixels can be reduced.

Now referring to FIGS. 4A and 4B, an advantage obtained by the circuitstructure of the first embodiment will be described. FIG. 4A shows therelationship between the applied voltage (data signal) and theillumination brightness for an example case where the organic EL elementis illuminated by the pixel circuit structure of the first embodimentshown in FIG. 2. Similarly, FIG. 4B shows the relationship between theapplied voltage (data signal) and the illumination brightness for anexample case wherein the organic EL element is illuminated by theconventional pixel circuit structure shown in FIG. 1. The setting inboth FIGS. 4A and 4B is such that the requested maximum brightness withrespect to the organic EL element occurs at the applied voltage (datasignal) of 8V, and the gradation display is performed at an appliedvoltage between 8V and 10V. The three samples in FIGS. 4A and 4Brespectively indicate illumination brightness characteristics for caseswherein organic EL panels having circuit structure respectively of FIG.2 and FIG. 1 are formed under different manufacturing conditions. Inother words, these samples indicate illumination brightnesscharacteristics for cases where the characteristic of the TFT in thepixel section is deliberately varied.

As is clear from FIGS. 4A and 4B, with the conventional circuitstructure, the brightness (luminance) characteristics for the threesamples having different characteristics for TFT in the pixel sectionsignificantly differ from each other at the set voltage range for datasignals between 8V and 10V. In contrast, with the circuit structureaccording to the first embodiment, although the characteristics differfrom each other at the high-brightness region to which human eyes areinsensitive, the brightness characteristic difference among the threesamples at the intermediate-brightness region is very small. Therefore,by employing a circuit structure as described in the first embodimentfor each pixel, even when the characteristic of the TFT is varied, inparticular, even when the characteristic of the EL element driving TFT20 which has a large influence is varied, the variation can becompensated by the compensation TFT 30 of an opposite polarity, thusenabling inhibition of the variation in the illumination brightness ofthe organic EL element.

FIG. 5 shows another example circuit structure according to the firstembodiment. The circuit structure of FIG. 5 differs from that of FIG. 2in that the second TFT 22 is constructed using an nch-TFT and thecompensation TFT 32 is constructed using a diode connected pch-TFT.Similar to the above structure, with this structure, the variation incharacteristic of the second TFT 22 can be compensated by thecompensation TFT 32.

FIG. 6 shows yet another example of a circuit structure according to thefirst embodiment. The circuit structure of FIG. 6 differs from that ofFIG. 2 in that a plurality of second TFTs are provided in parallelbetween the compensation TFT 30 and the organic EL element 50. Thepolarity of the TFTs are identical to that in FIG. 2, that is, thesecond TFTs 24 are of pch and the compensation TFT 30 is of nch. Thegates of two second TFTs 24 are both connected to the first TFT 10 andto the first electrode of the storage capacitor Cs. Each of the sourcesis connected to the compensation TFT 30 and each of the drains isconnected to the organic EL element 50. In this manner, by providing aplurality of the second TFTs 24 in parallel, it is possible to furtherreduce the variation in the current supplied to the organic EL elementdue to the characteristic variation of the second TFT.

If the target current value for each of the two second TFTs 24 isassumed to be i, the total target current value for the two second TFTs24 would be 2i. Even when the current supply capability for one of thesecond TFTs 24 is reduced to i/2 due to, for example, variations, theother second TFT 24 can continue to flow a current of i, and a totalcurrent of (3/2)i can be supplied to the organic EL element, where thetarget value is 2i. In the worst case, if the current supply capabilityof one of the TFTs becomes 0, a current of i can be supplied to theorganic EL element by the other TFT in the example shown in FIG. 6. Theadvantage of such a structure can be readily seen when one considers acase wherein the current supply capability becomes 0 in a circuit havingsingle second TFT 24 and the pixel becomes deficient.

Each TFT in the first embodiment is obtained by polycrystallizing ana-Si by a laser annealing process. When a plurality of second TFTs 24are provided in parallel, it is easy to arrange the positions of thesecond TFTs 24 so that the laser is not simultaneously irradiated toactive layers of both second TFTs 24 by, for example, shifting theformation positions with respect to the laser scan direction. Byarranging the second TFTs 24 in this manner, the probability that allsecond TFTS 24 become deficient can be significantly reduced, and thus,characteristic variation caused by the laser annealing can be minimized.In addition, as described above, because a compensation TFT 30 isprovided between the second TFTs 24 and the power supply line VL, evenwhen there is a shift in the threshold value of the second TFTS 24 dueto the variations in conditions such as, for example, annealingcondition, the shift can be alleviated by the compensation TFT 30.

FIG. 7 shows a further example pixel circuit structure according to thefirst embodiment. This circuit structure differs from that shown in FIG.6 in that not only are the second TFTs 24 provided in plurality, butthat the compensation TFTs are also provided in plurality. Eachcompensation TFT 34 is provided between the power supply line VL and thesecond TFTs 24. As shown in FIG. 7, by providing a plurality ofcompensation TFTs 34, variations in the current supply capabilitygenerated among the compensation TFTs 34 can be alleviated and, thus,reliability of reduction in variation in current supply capability tothe organic EL element 50 can be enhanced.

FIG. 8 shows one example of the planer structure of the organic ELdisplay device having a circuit structure shown in FIG. 7. FIG. 9A is aschematic cross section along the A—A line in FIG. 8, FIG. 9B is aschematic cross section along the B—B line in FIG. 8, and FIG. 9C is aschematic cross section along the C—C line in FIG. 8. In FIGS. 9Athrough 9C, the layers (films) that are simultaneously formed areassigned the same reference numeral except where their functions aredifferent.

As shown in FIG. 8, each pixel includes a first TFT 10, a storagecapacitor Cs, two pch-second TFTs 24, two nch-compensation TFTs 34 whichare diode-connected between the power supply line VL and the second TFTs24, and an organic EL element 50 connected to the drains of the secondTFTs 24. In the example of FIG. 8, although the configuration is notlimited to the one shown, a pixel is placed in the region surrounded bya gate line GL extending in the row direction and a power supply line VLand a data line DL both extending in the column direction. Also in theexample of FIG. 8, delta arrangement is employed for realizing a morehigh resolution color display device wherein the positions for R, G, andB pixels are shifted at each row, and consequently, the data line DL andthe power supply line VL are not straight, but extend in the columndirection through the gap between pixels having positions shifted foreach row.

In each of the pixel regions, the first TFT 10 is formed near the crosssection between the gate line GL and data line DL. As an active layer 6,p-Si obtained by polycrystallizing a-Si by a laser annealing process isused. The active layer 6 has a pattern where it steps over twice thegate electrode 2 which protrudes from the gate line GL. Although asingle gate structure is shown in FIG. 7, in the circuit, a dual gatestructure is employed. The active layer 6 is formed on a gate insulationfilm 4 which is formed to cover the gate electrode 2. The sections ofthe active layer 6 immediately above the gate electrodes 2 form channelsand source region 6S and drain region 6D to which an impurity is dopedare formed around the channels. Because it is desirable that the firstTFT 10 responds quickly to the selection signal output on the gate lineGL, impurity such as phosphorous (P) is doped into the source region 6Sand the drain region 6D, to form an nch-TFT.

The drain region 6D of the first TFT 10 is connected, via a contact holeopened in an interlayer insulation film 14 formed to cover the entirefirst TFT 10, to the data line DL formed on top of the interlayerinsulation film 14.

The source region 6S of the first TFT 10 is connected to the storagecapacitor Cs. The storage capacitor Cs is formed in the region where afirst electrode 7 and a second electrode 8 are overlapped with the gateinsulation film 4 in between. The first electrode 7 extends in the rowdirection in FIG. 8, similar to the gate line GL, and is formedintegrally with a capacitor line SL formed from the same material as thegate. The second electrode 8 is integral with the active layer(semiconductor layer) 6 of the first TFT 10 and is constructed by theactive layer 6 extending to the formation position of the firstelectrode 7. The second electrode 8 is connected to the gate electrodes25 of the second TFTs 24 via a connector 42.

The cross sectional structures for the two pch-second TFTs 24 and thetwo nch-compensation TFTs 34 are shown in FIG. 9B. The second TFTs 24and the compensation TFTs 34 use a semiconductor layer 16 patterned inan island-like manner for each TFT in the direction along the data lineDL (power supply line VL), as an active layer. Therefore, in thisexample, the channels of the second TFTs 24 and of the compensation TFTs34 are arranged so that the channel length direction is along the dataline DL, that is, along the longitudinal direction of the pixel havingan elongated shape. The semiconductor layer 16 is simultaneously formedwith the active layer 6 of the first TFT 10, and a polycrystallinesilicon formed by polycrystallizing a-Si by a laser annealing process isused as the semiconductor layer 16.

The compensation TFTs 34 placed at both ends of FIG. 9B are connected attheir respective drain region to the same power supply line VL via acontact hole opened in the interlayer insulation film 14. A gateelectrode 35 is provided immediately below the channel region of thecompensation TFT 34 with the gate insulation film 4 in between. The gateelectrode 35 is a layer formed by the same material as andsimultaneously with the gate line GL, and is connected to the powersupply line VL at a contact hole, as shown in FIG. 8. Therefore, thecompensation TFTs 34 construct diodes in which both gates and drains areconnected to the power supply line VL, as shown in the circuit diagramof FIG. 7. The source region of each compensation TFT 34 is provided tobe distant from the source region of the second TFT 24 constructed froma pch-TFT, and is connected to the source region of the second TFT 24 bya contact wiring 43.

Similar to the gate electrode 35 of the compensation TFTs 34, each gateelectrode 25 of the second TFTs 24 is a conductive layer formed from thesame material as and simultaneously with the gate line GL. Theconductive layer is connected to the second electrode 8 of the storagecapacitor Cs via the connector 42, extends from the formation region ofthe storage capacitor Cs along the power supply line VL, extends furtherunder the active layer 16, and forms each of the gate electrodes 25 ofthe two second TFTs 24.

The organic EL element 50 has a cross sectional structure as shown in,for example, FIG. 9C, and is formed on top of a flattening insulationlayer 18 provided over entire substrate for flattening the upper surfaceafter each of the TFTs are formed as described above. The organic ELelement 50 is constructed by laminating an organic layer between ananode (transparent electrode) 52 and a cathode (metal electrode) 57formed at the uppermost layer and common to all pixels. Here, the anode52 and the source region of the second TFT 24 are not directlyconnected, but are connected via a connector 40 which constructs awiring layer.

In the first embodiment, as shown in FIG. 8, two second TFTs 24 are bothconnected to a connector 40 and the connector 40 contacts the firstelectrode 52 of the organic EL element 50 at one contact point. In otherwords, the organic EL element 50 is connected to n second TFTs 24 atcontact points having the number equal to or smaller than (n−1). Becausethe contact region sometimes become a non-illuminating region, byminimizing the number of contact points between the organic EL element50 and the connector 40 (second TFTs 24), it is possible to maximize theillumination region. Another example related to the number of contactswill be described later as a third embodiment.

In the first embodiment, as shown in FIGS. 8 and 9C, the connectionposition between the connector 40 and the anode 52 is arranged so thatit is shifted from the connection position between the connector 40 andthe second TFTs 24. In the emissive element layer 51 which will bedescribed later and which includes an organic compound, electric fieldconcentration tends to occur at a locally thin region, and degradationmay be caused from the region where electric field concentrationoccurred. Therefore, it is desirable that the formation region of theemissive element layer 51 in which an organic material is used be asflat as possible. In the upper layer of a contact hole, a recess due tothe contact hole is produced, and the depth of the recess becomes largeras the contact hole becomes deeper. Therefore, by placing the contacthole for connecting the connector 40 and the source region of the secondTFTs 24 at a region outside the formation region of the anode 52, it ispossible to make the upper surface of the anode 52 onto which an organiclayer is formed as flat as possible. An example for flattening the uppersurface of the anode 52 will be described later to illustrate a fourthembodiment of the present invention.

The emissive element layer (organic layer) 51 comprises, from the sideof the anode, for example, a first hole transport layer 53, a secondhole transport layer 54, an organic emissive layer 55, and an electrontransport layer 56 laminated in that order. As an example, the firsthole transport layer 52 includes MTDATA:4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine, the second hole transportlayer 54 includesTPD:N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine,the organic emissive layer 55 includes, although dependent on the targetillumination color of R, G, and B, for example,BeBq₂:bis(10-hydroxybenzo[h]quinolinato)beryllium which includesquinacridone derivative, and the electron transport layer 56 isconstructed from BeBq. In the example of the organic EL element 50 shownin FIG. 9C, the layers (53, 54, 56, and 57) other than the anode 52constructed from an ITO (indium Tin Oxide) or the like and the organicemissive layer 55 are formed to be common to every pixel. Anotherexample of the structure of the EL element can be constructed bysequentially laminating the layers of (a) transparent layer (anode); (b)a hole transport layer constructed from NBP; (c) an emissive layerincluding red (R) constructed by doping a red dopant (DCJTB) into a hostmaterial (Alq₃), green (G) constructed by doping a green dopant(coumarin 6) into a host material (Alq₃), and blue (B) constructed bydoping a blue dopant (perylene) into a host material (BAlq); (d) anelectron transport layer constructed from Alq₃; (e) an electroninjection layer constructed from lithium fluoride (LiF); and (f)electrode (cathode) constructed from Aluminum (Al). The official namesof the above materials described in abbreviations are as follows:

-   “NBP”: N,N′-Di((naphthalene-1-yl)-N,N′-diphenyl-benzidine);-   “Alq₃”: Tris(8-hydroxyquinolinato)aluminum;-   “DCJTB”:    (2-(1,1-Dimethlethyl)-6-(2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl)-4H-pyran-4-ylidene)propanedinitrile;-   “coumarin 6”: 3-(2-Benzothiazolyl)-7-(diethylamino)coumarin; and-   “BAlq”:    (1,1′-Bisphenyl-4-0lato)bis(2-methyl-8-quinolinplate-N1,08)Aluminum.    The present invention, however, is not limited to these    configurations.

In a pixel having the structure as described above, when a selectionsignal is applied to the gate line GL, the first TFT 10 is turned on.The potential of the data line DL and the potential of the source regionof the first TFT 10 connected to the second electrode 8 of the storagecapacitor Cs become equal to each other. A voltage corresponding to thedata signal is supplied to the gate electrode 25 of the second TFT 24,and the second TFT 24 supplies, depending on the voltage value, acurrent to the anode 52 of the organic EL element 50, which is suppliedfrom the power supply line VL via the compensation TFT 34. With thisoperation, a current based on the data signal can be accurately suppliedto the organic EL element 50 for each pixel and, thus, uniform displaywithout variation can be achieved.

As shown in FIG. 8, because a plurality of (in this example, two)compensation TFTs 34 and second TFTs 24 are provided between the powersupply line VL and the organic EL element 50 in that order, when acharacteristic shift or deficiency is generated at one of these TFTs dueto a variation, the variation in the supplied amount of current, whichis determined by the total of the plurality of groups, is alleviated dueto the presence of the other TFT having normal characteristics.

In the planer placement shown in FIG. 8, a polycrystalline silicon layerproduced by polycrystallization by laser annealing process is used asthe active layers. The annealing process may be performed, for example,by scanning a laser beam which is longer in the row direction of thefigure, in the column direction. Even in such a case, the channeldirection of the first TFT 10 and the length channel direction of theactive layers of each of the second and compensation TFTs 24 and 34 donot coincide, and the formation positions for the first and second TFTs10 and 24 are far apart. Therefore, it is possible to preventsimultaneous generation of failures in the first and second TFTs 10 and24 and in the second and compensation TFTs 24 and 34 by the laserannealing.

In the above, all of the first TFT 10, second TFTs 24, and compensationTFTs 34 are described as a bottom gate structure, but these TFTs canhave a top gate structure wherein the gate electrode is formed on anupper layer than the active layer.

As described above, according to the first embodiment, it is possible toalleviate variations in characteristic of the transistor for supplyingpower to an element to be driven such as an organic EL element and,thus, it is possible to average the variation in the supplied power tothe element to be driven and to prevent variations in illuminationbrightness (luminance) or the like at the element to be driven.

Second Embodiment

A second embodiment of the present invention will now be described. Inthe first embodiment, in order to prevent variation in the illuminationbrightness among pixels as a result of characteristic variations in thetransistor, a compensation thin film transistor having an oppositeconductive characteristic as the element driving thin film transistor isprovided. In contrast, in the second embodiment, the variation in theillumination brightness among pixels is inhibited by considering theplacement of the element driving thin film transistor (second TFT).FIGS. 10A and 10B show an example configuration of one pixel accordingto the second embodiment. FIG. 10A is a schematic planer view and FIG.10B is a cross sectional view along the B—B line in FIG. 10A. Thisstructure is shown with the same circuit structure as that of FIG. 1. Inthese figures, the components corresponding to those in the drawingsthat are already explained will be referred to by the same referencenumerals.

In the second embodiment, one pixel comprises an organic EL element 50,a first TFT (switching thin film transistor) 10, a storage capacitor Cs,and a second TFT (element driving thin film transistor) 20. In contrastto the first embodiment, a single second TFT 20 is formed between thepower supply line VL and the organic EL element 50, and the second TFT20 is placed so that its channel length direction is along thelongitudinal direction of the pixel formed in an elongated shape,similar to the configuration shown in FIG. 8. In the second embodiment,by arranging the second TFT 20 so that the channel length direction isalong the longitudinal direction of the pixel region, the illuminationregion of the organic element 50 can be maximized, and, at the sametime, the necessary TFT can be efficiently placed in one pixel regionwhich has a limited area, even in the case where a second TFT 20 havinga long channel length is to be placed or in the case where a second TFT20 and a compensation TFT 30 must be placed between the power supplyline VL and the organic EL element 50 as shown in FIG. 8.

In the second embodiment, by placing the second TFT 20 in thelongitudinal direction of the pixel, the channel length of the secondTFT 20 can be lengthened to a sufficient length, as shown in FIGS. 10Aand 10B. By lengthening the channel length of the second TFT 20 to asufficient length, the reliability can be improved because thedurability of the TFT is improved. Moreover, this configuration enablesaveraging of the transistor characteristic of the second TFT 20, and,thus enables reduction in variations in the current supply capability ofthe second TFT 20 among pixels. The reduction of capability variationthen allows for significant reduction of the variation in theillumination brightness of the organic EL element 50 caused by such acapability variation.

In the second embodiment, the second TFT 20 uses a polycrystallinesilicon layer obtained by polycrystallizing an amorphous silicon layerby laser annealing as the semiconductor layer (active layer) 16, similaras in the first embodiment. In this case, by setting the scan directionof the laser annealing to coincide with the channel length direction ofthe second TFT 20, that is, by placing the irradiating region of thepulse laser so that the edge in the longitudinal direction crosses inthe width direction of the channel 16 c and by lengthening the channellength of the second TFT 20 as described above, the characteristicsvariation in the second TFT 20 can be reduced because it is easy toadjust the laser so that the entire channel region of the second TFT 20is not annealed by a single laser shot, and because generation of alarge difference in the characteristic of the second TFT 20 among otherpixels can be prevented. Thus, it is possible to obtain even higheraveraging effect on the characteristic of the second TFT 20.

It is desired that the second TFT 20 supplies a relatively large currentfrom the driving power supply (power supply line VL) to the organic ELelement 50. When a p-Si TFT which uses polycrystalline silicon for theactive layer 16 is used for the second TFT 20, the mobility of p-Si issufficient with respect to the desired capability, and, thus, the secondTFT 20 can achieve sufficient current supply capability even when thechannel length is designed to be lengthened. Because the second TFT 20is directly connected to the power supply line VL, the requireddurability is high, and consequently, it is often desired that thechannel length CL be longer than the channel width. Thus, in addition tothe above viewpoint, it is desirable that the channel length of thesecond TFT 20 be lengthened to a sufficient length. By forming thesecond TFT 20 so that the channel length direction is along thelongitudinal direction of the pixel region, the second TFT 20 with along channel can be efficiently placed within one pixel region.

In a display device constructed by arranging a plurality of pixels onthe display surface in a matrix form, the shape of each of the pixelstends to be designed to have a shape that is longer in the columndirection as shown in FIGS. 8 and 10A. In such a case, by placing thesecond TFT 20 so that the channel length direction is along the columndirection, the channel length would be along the longitudinal directionof the pixel region, and thus, the desired channel length as describedabove can be easily secured.

As shown in the second embodiment, in an active matrix type displaydevice wherein a switching element is provided in each pixel for drivingthe display element, a data line DL for supplying a data signal to thefirst TFT 10 is provided in the column direction and a selection line(gate line) GL is provided in the row direction. By placing the secondTFT 20 so that its channel length direction is along the extensiondirection of the data line DL (column direction), efficient placement ofthe second TFT 20 within the pixel region while securing a long channellength can be facilitated. In the example shown in FIGS. 10A and 10B, alayout wherein the power is supplied from a driving power supply Pvdd toeach pixel by the power supply line VL is employed. Because the powersupply line VL also extends in the column direction similar to the dataline, the channel length direction of the second TFT 20 also coincideswith the extension direction of the power supply line VL.

In the second embodiment, as described above, the second TFT 20 isarranged so that its channel length direction coincides with the scandirection of the laser annealing or is parallel to the column direction(extension direction of the data line DL), but the first TFT 10 isplaced so that its channel length direction coincides with the extensiondirection of the gate line GL, that is, the row direction. Thus, in thesecond embodiment, the first TFT 10 and the second TFT 20 have differentchannel length directions.

A cross sectional structure of the display device according to thesecond embodiment will now be described referring to FIG. 10B. FIG. 10Bshows a cross sectional structure of the second TFT 20 and the organicEL element 50 which is connected to the second TFT 20. The first TFT 10,which is not shown, has a basic structure similar to that of the secondTFT 20 shown in FIG. 10B, with the exceptions that the first TFT 10 hasa different channel length, a double gate structure, and a differentconductive type for the active layer 6.

The first TFT and second TFT shown in the first embodiment both have abottom gate structure, but the first TFT 10 and the second TFT 20 of thesecond embodiment have a top gate structure wherein the gate electrodeis formed on an upper layer than the active layer. The structure of thesecond embodiment is not limited to the top gate structure, and a bottomgate structure may also be employed.

The active layer 16 of the second TFT 20 and the active layer 6 of thefirst TFT 10 are both constructed from polycrystalline silicon obtainedby laser annealing and polycrystallizing an amorphous silicon layerformed on a substrate 1, as described above. A gate insulation film 4 isformed on top of the active layers 6 and 16 constructed formpolycrystalline silicon. Each of the gate electrodes 2 and 25respectively of the first TFT 10 and of the second TFT 20 is formed onthe gate insulation film 4. The gate electrode 25 of the second TFT 20is connected to the second electrode 8 of the storage capacitor Cs whichis integral with the active layer 6 of the first TFT 10. As shown inFIG. 10A, the gate electrode 25 is patterned so that it extends from theconnection section with the storage capacitor Cs in the column directionand widely covers the section of the gate insulation film 4 above theactive layer 16.

The region of the active layer 16 of the second TFT 20 which is coveredby the gate electrode 25 at the top is the channel region 16 c. A sourceregion 16 s and a drain region 16 d are formed at both sides of thechannel region 16 c. In the second embodiment, the source region 16 s ofthe active layer 16 is electrically connected to the power supply lineVL near the storage capacitor Cs via a contact hole formed to penetratethrough the gate insulation film 4 and the interlayer insulation film14. The drain region 16 d is connected to a connector (wiring layer) 40near the gate line GL which corresponds to the next row of the matrix,via a contact hole formed to penetrate through the gate insulation film4 and the interlayer insulation film 14. The connector 40 extends fromthe connection region with the drain region 16 d to the formation regionof the organic EL element 50, and is electrically connected to an ITOelectrode (anode) 52 of the organic EL element 50 via a contact holeformed on a first flattening insulation layer (planarizating insulationlayer) 18 which is formed to cover the interlayer insulation film 14,power supply line VL, and connector 40.

In FIG. 10B, only the central region of formation of the anode 52 of theorganic EL-element is opened above the first flattening layer 18. Asecond flattening (planarizating) insulation layer 61 is formed to coverthe edge of the anode 52, wiring region, and the formation regions forthe first and second TFTS. The emissive element layer 51 of the organicEL element 50 is formed on the anode 52 and the second flatteninginsulation layer 61. A metal electrode 57 which is common to all pixelsis formed on top of the emissive element layer 51.

The relationship between the channel length CL of the second TFT 20 andthe moving pitch P of the laser will now be explained. As describedabove, it is desired that the channel length CL of the second TFT 20 besufficiently long. In order to prevent annealing of the entire channelregion by one pulse laser, it is preferable to set the moving pitch P ofthe laser and the channel length CL so that P<CL. In some cases, themoving pitch P is adjustable according to the setting of the opticalassembly system of the laser annealing device or the like. In such acase, it is preferable that the device be adjusted so that CL>P. In adisplay device having a resolution of about 200 dpi, for example, evenwhen the length in the pixel row direction is about 30 μm, about 80 μmcan be secured in the column direction. Moreover, in a configurationwherein the moving pitch P of the laser is between 20 μm and 35 μm, byplacing the second TFT 20 so that its channel length direction is alongthe pixel longitudinal direction, a length of 50 μm to 80 μm can besecured as the channel length CL, and thus, the above relationship canbe satisfied. With such a relationship, the channel region 16 c of thesecond TFT 20 is always polycrystallized by a plurality of pulse laserirradiations, and it is possible to reduce the difference in thecharacteristic from the second TFT 20 of other pixels which aresimilarly polycrystallized by a plurality of pulse laser irradiations.

In the above explanation, a single second TFT 20 is formed between theorganic element 50 and the power supply line VL in one pixel. However,the second embodiment is not limited to such a configuration, and aplurality of second TFTs 20 may be provided in one pixel. FIG. 11 showsan example layout for a case wherein a plurality of second TFTs 20 areconnected in parallel between the power supply line VL and the organicEL element 50. The equivalent circuit of the pixel structure shown inFIG. 11 is similar to the case where the compensation TFT 30 is removedfrom the circuit shown in FIG. 6. The source regions 16 sa and 16 sb oftwo second TFTs 20 are both connected to the power supply line VL andthe drain regions 16 da and 16 db are both connected to the anode 52 ofthe organic EL element 50 via a contact 40 respectively. By providing aplurality of second TFTs 20 in one pixel in this manner, the probabilitythat no current can be supplied to the organic EL element because bothof the second TFTs 20 within one pixel simultaneously became deficientcan be reduced, at least to ½ or less.

The placement of two second TFTs 20 a and 20 b is such that the channellength direction of the second TFTs 20 a and 20 b is approximatelyparallel to the longitudinal direction (in this case, this directioncoincides with the extension direction of the data line DL) of the pixelregion similar to FIG. 10A. With such a placement, it is possible tomaximize the illumination region and, at the same time, to securemaximum length for each channel length CL. The scan direction of thelaser anneal is set, even in FIG. 11, to be parallel to both channellength directions of the two second TFTs 20 a and 20 b. The activelayers 16 a and 16 b are placed in a straight line. It is not necessarythat the active layers for a plurality of second TFTs 20 a and 20 b beprovided on a straight line, but because the channel regions 16 ca and16 cb of the second TFTs 20 a and 20 b do not completely coincide withthe laser scan direction and are somewhat shifted, it is possible tomore reliably prevent the characteristics of the TFTs 20 a and 20 b tovary in the same manner. In other words, because the channel lengthdirection is shifted from each other in the laser scan direction, theprobability that the channel for the two TFTs are simultaneouslyannealed by the same pulse is reduced and, thus, the probability of aproblem such as, for example, the characteristics of the second TFTs 20a and 20 b being shifted from the set value in the same manner orsimultaneous failure of both transistors can be significantly lowered.Therefore, the variation in the total amount of current supplied to theorganic EL element 50 among the pixels can be reduced.

It is preferable that both channel lengths CLa and CLb of the two secondTFTs 20 a and 20 b be greater than the moving pitch P of the laser, asdescribed above. Moreover, it is preferable that the separation distanceL between the channels 16 ca and 16 cb of the plurality of second TFTs20 a and 20 b be greater than the moving pitch P of the laser. However,when a plurality of second TFTs 20 is provided in one pixel, as shown inFIG. 11, simultaneous failure in the plurality of transistors (TFT) 2 aand 2 b within a pixel or characteristic shift in the same manner can beprevented and, thus, the reduction effect in the characteristicvariation among pixels can be achieved by at least setting the sum ofthe total channel length of the TFTs 20 a and 20 b and the separationdistance L to be larger than the moving pitch P.

As described above, according to the second embodiment, it is possibleto alleviate variations in characteristics of the transistor forsupplying power to an element to be driven such as an organic ELelement, and thus, it is possible to average the variation in thesupplied power to the element to be driven and to prevent variations inillumination brightness or the like at the element to be driven.

Third Embodiment

A method for efficiently connecting a plurality of second TFTs 20 andcorresponding organic EL element 50 within one pixel will now bedescribed as a third embodiment of the present invention. As describedin the first embodiment and as shown in FIG. 11 of the secondembodiment, provision of a plurality of second TFTs 20 between anorganic EL element 50 and a power supply line VL within one pixel isadvantageous from the viewpoint of improvements in reliability,characteristic, or the like. In a case wherein a plurality of secondTFTs 20 are provided within one pixel, as shown in FIG. 11, byrespectively connecting the second TFTs 20 a and 20 b and the organic ELelement 50, a current can more reliably be supplied from the powersupply line VL to the organic EL element 50 via the second TFTs 20.However, in an organic EL element of the type shown in FIG. 10B in whichlight from the emissive layer 55 is emitted from a transparent anode 52to the outside via the substrate 1 at the lower section, the contactsection usually has a light blocking characteristic. For example, inFIGS. 9C and 10B, the connection between the organic El element 50 andthe second TFT 20 is achieved by the wiring layer 40 (connector) whichis a metal wiring, and at the contact section between the wiring layer40 and the anode 52, the wiring layer 40 having a light blockingcharacteristic is present below the anode 52. Thus, in this region, thelight from the emissive layer 55 cannot pass through to the side of thesubstrate 1. Therefore, if n contact sections are provided between thesecond TFTs 20 and the organic EL element 50 to correspond to the nsecond TFTs 20, the illumination area would be reduced in proportion tothe number of contacts.

Therefore, in order to minimize the reduction in the illumination area,it is preferable to set the number of contacts between the second TFTs20 and the organic EL element 50 to be less than or equal to (n−1),wherein the number of second TFTS 20 in one pixel is n (n≧2). In FIG. 8and in FIGS. 12, 13, and 14 to be described below, n second TFTs 20 andthe organic EL element 50 are connected with the number of contactsbeing equal to or less than (n−1). In the figures to be explained below,the components that are common to the figures already described will beassigned the same reference numerals and will not be described again.

FIG. 12 shows a contact method between second TFTs 20 a and 20 b and anorganic EL element 50 when two second TFTs 20 a and 20 b are provided inparallel between the power supply line VL and the organic EL element 50.Similar to FIG. 11, the two TFTs 20 a and 20 b are placed such thatrespective channel length direction is parallel to the longitudinaldirection of the pixel (the extension direction of the data line DL) orto the scan direction of laser annealing. The TFTs 20 a and 20 b arealso placed so that they are shifted from each other. With such aconfiguration, as described above, the illumination variation amongpixels can be reduced and the reliability can be improved.

In the example shown in FIG. 12, a semiconductor layer constructed fromp-Si patterned into a single island-like manner is used for the activelayers 16 a and 16 b of the second TFTs 20 a and 20 b. The semiconductoris patterned so that both ends in the column direction are the sourceregions (in the case of a pch-TFT) 16 sa and 16 sb of respective secondTFTs 20 a and 20 b, and are connected to the power supply line VL. Theregion around the center of the semiconductor pattern defines the drainregions (in the case of a pch-TFT) 16 da and 16 db of the TFTs 20 a and20 b, and the drain regions are connected to a single wiring layer 40provided between the two TFTs via a common contact hole formed topenetrate through the interlayer insulation film 14 and the gateinsulation film 4 (refer to FIG. 10B).

The wiring layer 40 extends to the anode formation region of the organicEL element 50. Similar to the cross sectional structure shown in FIG.10B, the wiring layer 40 is connected to the anode 52 of the organic ELelement 50 via one contact hole opened on the first flatteninginsulation layer 18. Here, the connection position between the wiringlayer 40 and the anode 52 is set in FIG. 12 to be around the center ofthe anode 52 in the longitudinal direction of the pixel. The contactposition is not limited to the configuration of FIG. 12, but by placingthe contact position near the center of the anode 52 as shown in FIG.12, averaging effect of the current density can be obtained in theformation region of the anode 52 which is constructed from an ITO or thelike having a relatively high resistance compared to a metal electrodeand, thus, the uniformity of the illumination brightness at theillumination surface of each pixel can be improved.

In the example shown in FIG. 13, the number of second TFTs 20 isincreased to three. Three second TFTs 20-1, 20-2, and 20-3 are connectedin parallel between the power supply line VL and the anode of theorganic EL element 50. The active layer 16 of three second TFTs 20 areintegrally formed and are set so that the channel length direction isalong the row direction in FIG. 13. Each of the channel regions 16 c ₁through 16 c ₃ of second TFTs 20-1 through 20-3 are separated in theirchannel width directions by openings in the pattern of the active layer16.

Here, the three second TFTs 20 are connected to the power supply line VLat one contact point, and also connected to the anode 52 of the organicEL element 50 at one contact point by a single wiring layer 40. The gateelectrode 25 is common to all three TFTs, is electrically connected tothe second electrode 8 of the storage capacitor Cs, and is constructedfrom a metal wiring extending in the column direction from around thestorage capacitor Cs. In the configuration example of FIG. 13, threesecond TFTs 20-1 through 20-3 are connected to the organic EL element 50by one contact section. Therefore, the ratio of the occupational area ofthe contact section within the formation region of the organic ELelement 50 can be lowered, and thus, the ratio of opening in one pixel,that is, the illumination area, can be increased.

In an example shown in FIG. 14, the number of second TFTs 20 isincreased to 4. The four TFTs 20-1 through 20-4 are electricallyconnected in parallel between the power supply line VL and the anode 52of the organic EL element 50. The active layer 16 of four second TFTs 20are integrally constructed and the channel length directions of the TFTs20-1 through 20-4 are set to be parallel to the longitudinal directionof the pixel region or the extension direction of the data line DL,similar to FIG. 12. The four TFTs are arranged in an almost straightline.

Four TFTs 20-1 through 20-4 are connected to the power supply line VL atthree contact points, and connected to the anode 52 of the organic ELelement 50 at two contact points by first and second wiring layers 40-1and 40-2. In the example structure shown in FIG. 14, the source regions16S₁ and 16S₄ of the TFTs 20-1 and 20-4 which are located at theoutermost positions of the single active layer 16 are respectivelyconnected to the power supply line VL as a separate entity. The sourceregions 16S₂ and 16S₃ of the TFTs 20-2 and 20-3 which are located at thecentral position are together connected to the power supply line VL. Thesecond TFTs 20-1 and 20-2 and the organic EL element 50 are connected asfollows. The drain regions 16 d ₁ and 16 d ₂ of the second TFTs 20-1 and20-2 are connected to a first wiring layer 40-1 extending from betweenthe second TFTs 20-1 and 20-2 to the element 50, and the first wiringlayer 40-1 extends to the formation region of the organic EL element 50and is connected to the anode 52 of the element. The second TFTs 20-3and 20-4 are connected to the organic EL element 50 as follows. Thedrain regions 16 d ₃ and 16 d ₄ of the second TFTs 20-3 and 204 areconnected to a second wiring layer 40-2 extending from between thesecond TFTs 20-3 and 20-4 to the element 50, and the second wiring layer40-2 extends to the formation region of the organic EL element 50 and isconnected to the anode 52 of the element. In this manner, four secondTFTs 20-1 through 20-4 are connected to the organic EL element 50 onlyat two contact points, in order to inhibit the reduction of illuminationregion caused by providing four second TFTs 20-1 through 20-4.

In the configuration of FIG. 14, because the four second TFTs 20-1through 20-4 are placed so that the channel length direction is directedalmost in a straight line along the longitudinal direction of the pixel,it is possible to efficiently place the second TFTs 20-1 through 20-4within one pixel.

As described above, according to the third embodiment, by connecting anelement to be driven and a transistor for supplying power to the elementby minimum number of contacts, necessary transistors and elements can beefficiently placed in a limited area. Therefore, when an EL element isused, for example, as the element to be driven, the illumination arearatio can be improved in one pixel unit.

Fourth Embodiment

A connection structure between the second TFT 20 and the organic ELelement 50 will now be described referring to FIGS. 15 through 20. Asdescribed in the third embodiment, in a device in which light istransmitted through a transparent anode 52 and emitted outside from thelower substrate 1 (bottom emission), the contact region between theorganic EL element 50 and the second TFT 20 is usually anon-illuminating region. Also, in order to improve the integrationdensity in many integrated circuits, and in order to improve theresolution in a display device, it is desired to minimize the contactarea. From such a viewpoint, when the active layer 16 of the second TFT20 is directly connected to the anode 52 of the organic EL element 50 orwhen the direct connection is not employed and a metal connection layer(Al layer, Cr layer or the like) is provided in between for improvingthe connection characteristic, it is preferable to form the firstcontact hole 70 of the interlayer insulation film 14 and the secondcontact hole 72 of the first flattening insulation layer 18 to overlapeach other, as shown in FIGS. 15A and 15B.

However, when a plurality of contact holes are formed to overlap eachother as shown in FIG. 15A, the total step size (h70+h72) of the contactholes become large and, thus, the surface flatness of the layer formedon top of the contact hole is reduced. Moreover, there are some caseswhere a second flattening insulation layer 61 is used for covering theedge region of the anode 52 as shown in FIG. 15A in order to preventshortage between the anode 52 and the cathode 57 caused by coveragefailure of the emissive element layer 51 at the anode edge region. Thesecond flattening (planarizating) insulation layer 61 is opened at thecentral region of the anode 52. Therefore, the opened section of thesecond flattening insulation layer 61 is formed near the first andsecond contract holes 70 and 72, and the formation surface of theemissive element layer 51 will be influenced also by the step h74 causedby the opening of the second flattening insulation layer 61.

In the organic EL element 50, on the other hand, illumination organiccompound in the emissive layer 55 is illuminated by flowing a currentthrough the emissive element layer 51. It is known that if there is alarge difference in the thickness within the layer of the emissiveelement layer 51, an electric field concentration tends to occur at aportion that is thinner than the other portions, and a dark spot tendsto be generated at such a portion. Dark spots degrade display quality,and furthermore, because dark spots tend to expand as the element isdriven, each dark spot shortens the life of the element. Therefore, whenthe organic EL element 50 is formed at a layer above the contact region,it is desired to maximize the flatness of the formation surface of theemissive element layer 51. The contact structure shown in FIGS. 15A and15B in which the emissive element layer 51 is formed on an unevensurface is not desirable from the viewpoint of improving the reliabilityof the emissive element layer 51.

In consideration of the above, FIGS. 16A and 16B show an example of aconnection method wherein the flatness at the formation surface of theemissive element layer 51 is increased, considering the above. FIG. 16Ashows a cross sectional structure of the contact section between theactive layer 16 of the second TFT 20 and the anode 52 of the organic ELelement 50. FIG. 16B shows a schematic planer structure of the contactsection. With exception of the presence of the second flatteninginsulation layer 61 for covering the edge region of the anode 52 and thesecond TFT being a top gate structure, the connection structure shown inFIGS. 16A and 16B is identical to the structure shown in FIGS. 8 and 9as explained for the first embodiment. The connection position betweenthe wiring layer 40 and the anode 52 is placed such that it is shiftedwith respect to the connection position between the wiring layer 40 andthe active layer 16 of the second TFT 20. By employing such aconfiguration, the anode surface at the contact region between thewiring layer 40 and the anode 52, being the formation surface of theemissive element layer 51, is only influenced by the step h72 caused bythe second contact hole 72 and is not influenced by the step h70 causedby the first contact hole 70 as in the case shown in FIGS. 15A and 15B.Therefore, as is clear from comparison between FIGS. 15A, B and 16A, B,the flatness of the formation surface for emissive element layer,especially at the illumination region of each pixel onto which theemissive layer 55 is formed can be improved.

FIG. 17 shows a method for further flattening the formation surface ofthe emissive element layer as shown in FIG. 16A. In the example shown inFIG. 17, similar to FIG. 16A, the formation position of the secondcontact hole 72 for connecting the wiring layer 40 and the anode 52 ofthe organic EL element 50 is shifted from the formation position of thefirst contact hole 70, and, in addition, the second contact hole 72 iscovered by the second flattening insulation layer 61. Therefore, theregion onto which the emissive layer 55 is formed is neither influencedby the step caused by the first contact hole 70 nor by the step causedby the second contact hole 72. The flatness of the formation surface ofthe emissive element layer can thus be further improved. Because thesecond flattening layer 61 covers the edge region of the anode 52,shortage between the anode 52 and the cathode 57 or the like canreliably be prevented.

The illumination region of the organic EL element is a region in whichthe anode 52 and the cathode 57 oppose each other with the emissivelayer 55 in between, and the region in which the second flatteninginsulation layer 61 is formed between the anode 52 and the emissiveelement layer 51 does not illuminate. Therefore, strictly speaking, withthe configuration shown in FIG. 17, because the second flatteninginsulation layer 61 covers not only the edge of the anode 52 but alsothe section above the second contact hole 72, the illumination region isreduced. However, as described above, when the wiring layer 40 or thelike which has a light blocking characteristic is formed at a lowerlayer, the formation region of the wiring layer 40 will be anon-illuminating region when seen from outside. Therefore, even when thestructure shown in FIG. 17 in which the second flattening insulationlayer 61 covers the second contact hole 72 is employed, the actualreduction in the illumination area due to the formation of the secondflattening insulation layer 61 for one pixel can be inhibited.

The improvement effect on the flatness of the formation surface for theemissive element layer can also be achieved by the method of coveringthe contact hole by the second flattening insulation layer 61, byemploying a layout wherein the first and second contact holes 70 and 72are placed to overlap each other, as in FIGS. 15A and 15B. Specifically,as in the cross sectional structure of the contact section shown in FIG.18, the active layer 16 of the second TFT 20 and the anode 52 of theorganic EL element 50 are connected by first and second contact holes 70and 72 formed to overlap each other, and the region of the anode 52having the upper surface recessed because of two contact holes iscovered by the second flattening insulation layer 61. The formationsurface for the emissive element layer above the contact holes 70 and 72will thus be a surface with a good flatness, formed by the secondflattening insulation layer 61. Also, by placing the two contact holes70 and 72 at the same position in FIG. 18, the element placementefficiency in one pixel is high and it is easy to contribute to animprovement in the illumination region.

FIG. 19 shows another flattening method of the formation surface for theemissive element layer. The method of FIG. 19 differs from that of FIG.17 in that a filling layer 62 is selectively formed on top of the anode52 instead of the second flattening insulation layer 61, at theformation region of the second contact hole 72, in order to fill therecess caused by the contact hole. By selectively forming the fillinglayer 62 on top of the anode 52 for covering the contact hole 72, evenwhen the second flattening insulation layer 61 or the like is notprovided, the formation surface for the emissive element layer above thecontact hole can be flattened. As shown in FIG. 20, the filling layer 62may also be employed, similar to FIG. 19, for a case wherein the firstand second contact holes 70 and 72 are formed to overlap each other. InFIG. 20, the filling layer 62 is selectively formed on the anode 52 atthe region where the two contact holes are formed to overlap, in orderto fill the deep recess formed by the two contact holes. In the casesillustrated in FIGS. 19 and 20, the emissive element layer 51 is formedon a flat surface of the filling layer 62 at the region where thecontact hole or contact holes are formed and, thus, failures in theemissive element layer generating in this region can be prevented.

The material for the second flattening insulation layer 61 and for thefilling layer 62 can be any material which has a flat upper surface, butit is preferable to use a stable and insulating material which does notreact with the emissive element layer 51 and which is not hydrous. Forexample, polyimide, HMOSO, TOMCAT, or TEOS can be used.

As described above, according to the fourth embodiment, the flatness ofthe surface onto which the element to be driven such as an organic ELelement, is formed can be improved, and thus, it is possible to improvethe reliability of the element to be driven.

1. A semiconductor device comprising: a switching thin film transistorwhich operates by receiving a gate signal at its gate and for reading adata signal; and an element driving thin film transistor providedbetween a driving power supply and an element to be driven, forcontrolling the power supplied from said driving power supply to saidelement to be driven based on a data signal supplied from said switchingthin film transistor; wherein a compensation thin film transistor havingan opposite conductive characteristic with respect to said elementdriving thin film transistor is provided between said driving powersupply and said element driving thin film transistor, wherein a gate andeither a source or a drain of said compensation thin film transistor areconnected, and said compensation thin film transistor is connectedbetween said driving power supply and said element driving thin filmtransistor.
 2. A semiconductor device according to claim 1, wherein saidelement driving thin film transistor comprises a plurality of thin filmtransistors connected to each other in parallel.
 3. A semiconductordevice according to claim 2, wherein said compensation thin filmtransistor is a diode connected transistor connected between saiddriving power supply and said element driving thin film transistor.
 4. Asemiconductor device according to claim 1, wherein said element drivingthin film transistor comprises a plurality of thin film transistorsconnected between said driving power supply and said element to bedriven and in parallel to each other; and said compensation thin filmtransistor is respectively provided between said plurality of thin filmtransistors connected in parallel and said driving power supply.
 5. Asemiconductor device according to claim 4, wherein said compensationthin film transistor is a diode connected transistor connected betweensaid driving power supply and said element driving thin film transistor.6. A semiconductor device according to claim 1, wherein said element tobe driven is an electroluminescence element which includes an emissivelayer between a first electrode and a second electrode.
 7. Asemiconductor device according to claim 5, wherein saidelectroluminescence element is an organic electroluminescence elementwhich employs an organic compound in an emissive layer.
 8. Asemiconductor device according to claim 1, wherein said semiconductordevice is used for an active matrix type display device wherein each ofthe pixels arranged in a matrix form comprises said switching thin filmtransistor, said element driving thin film transistor, said compensationthin film transistor, and said element to be driven as a displayelement.
 9. A semiconductor device according to claim 1, wherein saidelement driving thin film transistor and said compensation thin filmtransistor are placed so that the channel length direction of said thinfilm transistors is along the extension direction of the data line forsupplying said data signal to said switching thin film transistor.
 10. Asemiconductor device according to claim 1, wherein the channel lengthdirection of said element driving thin film transistor does not coincidewith the channel length direction of said switching thin filmtransistor.
 11. A semiconductor device comprising: a switching thin filmtransistor which operates by receiving a gate signal at its gate and forreading a data signal; and an element driving thin film transistorprovided between a driving power supply and an element to be driven, forcontrolling the power supplied from said driving power supply to saidelement to be driven based on a data signal supplied from said switchingthin film transistor; wherein a compensation thin film transistor havingan opposite conductive characteristic with respect to said elementdriving thin film transistor is provided between said driving powersupply and said element driving thin film transistor, wherein saidelement driving thin film transistor is formed so that its channellength direction is along the scan direction of a line pulse laser forannealing the channel region of the transistor.
 12. A semiconductordevice according to claim 11, wherein a gate and either a source or adrain of said compensation thin film transistor are connected, and saidcompensation thin film transistor is connected between said drivingpower supply and said element driving thin film transistor.